Responsive design means the site automatically adapts to screen size and works equally well on a phone, tablet and desktop. It isn't an add-on, it's the baseline for any modern website.
Also known as: Mobile optimisation, Mobile-friendliness, Responsiveness
Most of a small business's customers browse on a phone, often at the exact moment they decide to buy or get in touch. A site that doesn't work on mobile loses those customers outright.
Google judges sites mainly by how they look on a phone, not a desktop. A site that performs poorly on mobile ranks lower in search results, even for searches done on a desktop.
Written by Eetu Huotari